The overall goal of this research is to define alterations in myocardial transmembrane distribution of sodium and alterations in high-energy phosphate metabolism preceeding and during disturbances in cardiac rhythm which may be causal in sudden cardiac death. That alterations in intracellular ion concentrations are an important determinant of arrhythmias is evidient from instances of ventricular fibrillation which accompany digitalis intoxication. The relationship between energy metabolism and alterations of intracellular ion concentrations in cardiac tissue may be expected to be important as ATP is the energy source for maintenance of transmembrane ion gradients. We will approach this problem through the study of isolated perfused working guinea pig hearts by phosphorus-31 and sodium-23 nuclear magnetic resonance.